1. Field of the Invention
The present invention relates to a composite aluminum member joining process. The composite aluminum member joining process according to the present invention can be employed in the manufacture of an automobile internal combustion engine. Here, the composite aluminum member means one comprising a member made of a pure aluminum expanded material or an aluminum alloy expanded material and a member made of an aluminum alloy casting joined thereto. Hereinafter, the member made of a pure aluminum expanded material or an aluminum alloy expanded material is simply referred to as an aluminum member, and the aluminum member includes a preformed member, an expanded member, an extruded member and so on made of pure aluminum or aluminum alloys.
2. Discussion of the Prior Art
Since an aluminum member is covered with an oxide film formed closely and strongly thereon, it is therefore very hard to join or weld an aluminum member to a member made of an aluminum alloy casting (or a cast member made from a molten aluminum alloy, hereinafter simply referred to as an aluminum alloy cast member).
It has been known to coat an aluminum member with a flux comprising potassium tetrafluoro aluminate (KAlF.sub.4) in advance so as to avoid the difficulty in joining the aluminum member to an aluminum alloy cast member. Further, another method has been proposed in Japanese Unexamined Patent Publication No. 33752/1986, in which a chemical film comprising potassium pentafluoro aluminate (K.sub.2 AlF.sub.5.H.sub.2 O) is formed on an aluminum member, and thereafter the aluminum member is brought into contact with and joined or welded to an aluminum alloy cast member made from a molten aluminum alloy.
These methods have been devised in order to improve the wettability between an aluminum member and a molten aluminum alloy. However, the methods have not been applicable to a casting process employing a disposable pattern, because the disposable pattern generates tar-like substances when it comes in contact with the molten aluminum alloy, burns and vaporizes. The tar-like substances remain on the boundaries between an aluminum member and an aluminum alloy cast member made from a molten aluminum alloy, and accordingly they prevent the molten aluminum alloy from coming in contact with the aluminum member. The tar-like substances thus hinder the joining of an aluminum member and an aluminum alloy cast member made from a molten aluminum alloy.